Method of producing superconducting strips

ABSTRACT

A method of producing a superconducting strip, comprising continuously passing a superconducting wire or tape through a mold, casting a molten stabilizing metal for said wire into said mold, cooling the molten stabilizing metal during its passage through said mold along the travelling path of said wire with said wire embedded therein to thereby solidify said molten stabilizing metal in close contact with said wire and continuously drawing the resultant superconducting strip from said mold; and an apparatus for practicing said method. The superconducting strip produced according to the present invention is free from breakage of the wire, has highly uniform and excellent properties and can be produced at lower cost than the conventional ones.

United States Patent 1 1 Doi et al.

1 51 Jan. 16,1973

[54] METHOD OF PRODUCING SUPERCONDUCTING STRIPS Assignee: Hitachi, Ltd., Tokyo, Japan Filed: Feb. 20, 1968 Appl. No.: 706,924

[30] Foreign Application Priority Data [56] References Cited UNITED STATES PATENTS 2,959,829 11/1960 Brennan ..l64/86 X 3,317,286 5/1967 DeSorbo ....29/l83.5 3,243,871 4/1966 Saur ..29/155.S

Carlson 29/ l 94 Grubessich et al ..1 17/1 14 B Primary ExaminerJ. Spencer Overholser Assistant Examiner-V. K. Rising AttorneyCraig, Antonelli and Hill [57] ABSTRACT A method of producing a superconducting strip, comprising continuously passing a superconducting wire or tape through a mold, casting a molten stabilizing metal for said wire into said mold, cooling the molten stabilizing metal during its passage through said mold along the travelling path of said wire with said wire embedded therein to thereby solidify said molten stabilizing metal in close contact with said wire and continuously drawing the resultant superconducting strip from said mold; and an apparatus for practicing said method. The superconducting strip produced according to the present invention is free from breakage of the wire, has highly uniform and excellent properties and can be produced at lower cost than the conventional ones.

2 Claims, 9 Drawing Figures PATENTEDJAN 16 I975 3.710.844

SHEET 1 OF 6 INVENTORY TO-SVI/O 30/ ,wrzs-a mac have ATTORNEYS PATENTEU JAN 1 6 I975 SHEET 2 [IF 6 FIG. 4

' Nb 25zr H (KOe) INVENTOR BY Y QM ATTORNEYS PATENTEDJAH 16 I975 3.710.844

INVEN1OR TOSH/O .70/

N/TSUH/R K 4 ATTORNEYS PATENTEUJAH 16 I975 SHEET 6 0F 6 FIG. 8

600 700 800 900 /O00 //00 I200 I300 H547 TR547'ME/VT TEMPERATURE "C INVENTOR TOSH/ 0 30/ M/T-SzI/I/R K4190 ATTORNEYS METHOD OF PRODUCING SUPERCONDUCTING STRIPS BACKGROUND OF THE INVENTION The present invention relates to a novel method for producing a superconducting strip composed of a superconducting wire having a stabilizing metal intimately bonded to the peripheral surface thereof.

Superconducting wires used as winding of a large superconducting magnet or transformer have a structure in which a wire or a tape of superconducting material (which is hereinafter referred to as superconducting wire or simply as wire) is stabilized by a largeamount of a material, such as copper, casted over the peripheral surface thereof, that is, the structure which is usually referred to as a strip or a cable."

The superconducting strips of such structure have heretofore been produced, for example, by embedding in a copper strip a superconducting wire which has previously been subjected to final heat treatment for increasing the critical current (Ic value), rolling said copper strip with the superconducting wire embedded therein and annealing the rolled product to further strengthen the bonding between said copper strip and said wire as well as to reduce the strain of copper resulting from rolling.

By coating the peripheral surface of the superconducting wire with a large amount of copper in the manner described, the local normal state causedby the so-called Flux Jump is stabilized, due to the current by-passing action and the heat dissipating action of the copper. Consequently, the superconducting strip when wound into a coil is capable of flowing a current therethrough which is approximately the same as the critical current for a short sample of the wire.

The strip used for the production of a superconducting strip may consist, instead of copper, of a material such, for example, as Au, Ag, Cd, Al, In, Sn or Pb, which has substantially the same resistivity as that of copper at temperatures below the critical temperature of the wire used. The material which is intimately bonded to the superconducting wire for the stabilization of said wire, is called a stabilizing metal.

The conventional method of producing a superconducting strip as described above, however, has several fundamental defects. Namely, firstly, the superconducting wire which is mechanically frail, tends to be broken or locally reduced in diameter due to an excessive pressure exerted thereon during rolling of the copper strip with said wire embedded therein, and as a result, the characteristics of the produced superconducting strip are extremely deteriorated; secondly, the annealing which follows the operation of embedding the wire in the copper strip frequently results in degradation of the superconducting characteristics of the wire, e.g. lowering of the critical current; and thirdly, it is impossible to obtain a superconducting strip of a great length and uniform structure due to the defect set out in the first place here above.

SUMMARY OF THE INVENTION An object of the present invention is to improve the bonding between the superconducting wire and the stabilizing metal and to improve the property and uniformity of the produced superconducting strip during the production process. Another object of the invention is to reduce production costs.

According to the present invention, there is provided a method of producing a superconducting strip, comprising continuously passing a superconducting wire through a mold having the same transverse cross sectional shape as that of said superconducting strip, casting a molten stabilizing metal for said wire into said mold, cooling the molten stabilizing metal during its passage in said mold along the travelling path of said wire to thereby solidify said molten stabilizing metal with said wire embedded therein and continuously drawing the produced superconducting strip from said mold.

In operating the method of this invention, the superconducting wire is heated to a temperature at least higher than the melting point of the stabilizing metal and such heating brings about a heat treatment effect which advantageously affects the characteristics of the produced superconducting strip when said superconducting strip comprises a combination of a specific superconducting wire and a specific stabilizing metal.

The method of this invention is highly effective in the production of superconducting strips using not only alloy-type superconducting materials, such as Nb-Zr, Nb-Ti and Nb-Zr-Ti alloys, but also compound-type superconducting materials, such as Nb Al, Nb Sn and V Ga. As a stabilizing metal, metals are used which have a relatively low melting point and a good bonding ability to the above-mentioned superconducting materials and which have a sufficiently low magnetoresistance and high thermal as well as electrical conductivities at a temperature in the proximity of the critical temperature. In practice, Cu, Ag, Au, Cd, Al, In, Sn and Pb are advantageously used. Aluminum is particularly advantageously used as a stabilizing metal because it has an extremely low magneto-resistance at a temperature below the critical temperature. In the past, use of aluminum involved a difficulty in respect of bonding with the superconducting materials but, according to the present invention, such difficulty can be readily eliminated and the bonding property of aluminum can be improved remarkably, because it is bonded directly with the wire in a molten state.

The method of the present invention can be practiced by some of the apparatus similar to the conventional continuous casting apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view schematically showing one form of the continuous casting apparatus to be used in the present invention;

FIG. 2 is a top end view of the mold shown in FIG. 1;

FIG. 3, comprising FIG. 3a and FIG. 3b, is a set of FIG. 7 is a chart illustrating the H-lc characteristic curve of a superconducting strip produced according to- DESCRlPTlON OF THE PREFERRED EMBODIMENTS Referring to FlG. 1, there is shown schematically an apparatus for the continuous casting of a superconducting wire with a stabilizing metal, which is preferably used for practicing the method of this invention. In the apparatus shown, superconducting wires 1 and l, stored on respective spools 2 and .2, are paid out therefrom as required and passed through surface cleaning tanks 3 and 3' respectively, wherein the grease and other impurities present on the surfaces of said wires are removed therefrom by a chemical method such, for example, as electrolytic polishing or washing with acid, or by a mechanical method such as ultrasonic cleaning.

The superconducting wires 1 .and 1' leaving the respective surface cleaning tanks 3 and'3 are trailed around respective guide rolls 4 and 4' and led into a mold 6 after passing through a positioning guide 5 which is fixedly held above said mold 6 and by which the relative positions of the wires 1 and l to the mold 6 are adjusted in such a way that said wires 1 and 1' respectively pass in a predetermined path within the mold 6. The mold 6 is provided with a cooling jacket 13 through which cooling water or cooling air is circulated. A molten metal reservoir 7 containing a molten stabilizingmetal 8 is arranged above the mold 6 and from which the molten stabilizing metal 8 is poured into the mold 6. Thus, it will be understood that the superconductive wires 1 and l' are embedded in the molten stabilizing metal 8 during passage through said mold and said molten stabilizing metal 8 is cooled and solidified by the cooling water or cooling air circulating through the cooling jacket 13. The web of superconducting strip 9 thus produced is continuously drawn from the mold 6 by counter-rotating rolls l0 and taken up on a winding spool 11. Between the rolls l0 and the mold 6 may be provided cooling water sprays 12 as required for quick cooling of the superconducting strip 9. The superconducting wires 1 and 1' being led into the mold 6 may be given a tension to some extent so as to prevent said wires from swerving in said mold. The molten stabilizing metal 8 is continuously poured into the mold 6 at an optimum rate through a regulating valve 15, while the molten metal reservoir 7 is supplied with the molten metal-through a molten metal inlet 14 continuously to make up for the amount used. Means for holding the above-mentioned parts of the apparatus in their respective positions and means for driving the spools and rolls are not shown in the Figure as they are not directly pertinent to the present invention.

In pouring the molten metal into the mold, care must be exercised so as not to produce a strain in the wires and not to form a cavity or cavities-between the wires and the molten metal. Namely, when the wires are in the form, for example, of a tape as indicated by numerals 21, 21' in FIG. 2 which i's'atopplan view of one form of the mold 6, the molten metal should be poured from the side edges of the tapes 21, 21 in a direction as indicated by the arrow. It is also effective in improving the bond between the wires and the molten metal to pour the molten metal while vibrating said mold.

According to the present invention, superconducting strips of a variety of shapes and structures can be obtained by merely exchanging the mold and the positioning guide respectively in whatever configuration or structure. Examples of modifications of the positioning guide are shown in FIGS. 3a and 3b. The positioning guide shown in FIG. 3a is adapted for use in the production of a superconducting strip comprising a single tape-like wire embedded in the stabilizing metal, whereas the positioning guide shown in FIG. 3b is adapted for use in the production of a superconducting strip comprising 10 fine wires embedded in the stabilizing metal.

The superconducting characteristics (such as critical current) of a superconducting wire or a superconducting strip are generally variable largely depending upon the degree of mechanical work and heat treatment, to which the superconducting material used is subjected. The ordinary superconducting materials each have their own suitable heat treatment temperature range and heating period. Therefore, in practicing the present invention, use of a stabilizing metal whose melting point is significantly higher than the heat treatment temperature suitable for the superconducting wire used, should be avoided. On the other hand, when the melting point of an ordinary conducting material to be used for casting is substantially the same as the temperature, used for the final heat treatment which follows the casting operation, and suitably selected for the superconducting wire used, said final heat treatment may be eliminated partially or entirely by effectively making use of the heat of the molten metal, poured into the mold, for heating the wire.

As an example, heat treatment of a Nb-Zr binary alloy is preferably effected in the temperature range of 350 to 700 C. for about 10 hours at lower temperatures and for about 30 minutes at higher temperatures, though slightly variable depending upon the mixing ratio of the constituent metals. Therefore, in this case, Al, Sn, Pb, Cd andln can be used as a stabilizing metal. In particular, when molten aluminum at a temperature lower than 800 C is used as a stabilizing metal, the step of the aforementioned final heat treatment can be eliminated partially by cooling the molded strip over a relatively long period.

Further, when a stabilizing metal is used whose melting point is relatively low with respect to the final heat treatment temperature optimum of the wire used, it is possible to avoid substantial changes in the characteristics of the wire, by previously subjecting said wire to heat treatment under the optimum final heat treatment conditions, even though the wire may be heated to some extent by said molten metal.

FIG. 4 is a chart illustrating the H-lc (magnetic fieldcritical current) characteristic curve of a stip produced according to the present invention using a wire of Nb- 25 at. Zr (hereinafter referred to as Nb-25Zr for simplicity), and in which is illustrated the amount of currentper unit cross sectional area of the wire in a magnetic field. Curve Akin the chart represents the characteristics of the Nb-25Zr wire before the heat treatment,

and curve B represents the characteristic curve of a strip which is produced by subjecting the same wire as mentioned above to heat treatment at 500 C for 60 minutes, casting molten aluminum to it at 700 C and cooling thus obtained strip to the normal temperature in about 10 minutes. Curve C represents the characteristics of a strip which is produced by casting molten tin over the same wire as mentioned relating to the curve A, after the wire has been heat treated at 700 C for 60 minutes. From this chart, it will be seen that while the critical current lc of the non-heat treated wire of 0.25 mm in diameter is only of the order of 25A at most in the magnetic field of 40 KOe, the current capacity of the strips according to the present invention is as large as about 50Av The strips of this invention which are produced in the same manner as the strip of curve C but casting Cd, Pb and In at 350 C individually, instead of tin, have characteristics which are substantially the same as curve C.

Heat treatment of Nb-Ti type binary alloys or the socalled Nb-Ti side Nb-Zr-Ti type ternary alloys in which Ti is relatively large in amount with respect to Zr, are preferably carried out in the temperature range of 300 to 600 C for about l hours at lower temperatures and for about minutes at higher temperatures,'though slightly variable depending upon the mixing ratio of the constituent metals. Therefore, in the production of a strip according to the method of this invention using a wire of Nb-Ti type binary alloyvor Nb-Ti side Nb-Zr-Ti ternary alloy, it is preferable to'use as a stabilizing metal, such a metal as Sn, Pb or Cd, whose melting point is relatively low.

The chart of FIG. 5 shows the H-lc characteristic curve of a superconducting strip produced according to the present invention using a superconducting wire of Nb-65Ti alloy. In the chart, curve D represents the characteristic of the wire before heat treatment and curve E represents the characteristic of a strip which is produced according to the present invention by subjecting the wire to heat treatment at 500 C for 60 minutes beforehand and casting tin as a stabilizing metal at 300 C.

Of the superconducting materials consisting of Nb- Zr-Ti type ternary alloy, the so-called Nb-Zr side ternary alloys which contain Zr in a larger amount than Ti may be heat treated under substantially the same conditions as those for the aforesaid Nb-Zr type binary alloy. The chart of FIG. 6 shows the H-lc characteristic curve of a superconducting strip which is produced according to the present invention using a wire of Nb- 40Zr-l0Ti ternary alloy. Curve F in the chart represents the characteristic of the wire before heat treatment; curve G represents the characteristic of a strip which is producedby casting Al at 700 C over the I wire which has previously been subjected to heat treatment at 500 C for 60 minutes, and cooling thus produced strip to the normal temperature in 10 minutes; and curve J represents the characteristic of a strip which is produced by casting Cd at 350 C over the wire which has previously been subjected to heat treatment at 550 C for 60 minutes. Curve K represents the characteristic of a strip which is produced according to the aforementioned conventional method by subjecting the wire to heat treatment at 550 C for 60 minutes, embedding said wire in a copper strip mechanically, and after rolling, annealing thus obtained strip at 500 C for l0 hours,

The H-lc characteristic of the wire which has been subjected to heat treatment at 550 C for minutes, is substantially the same as that represented by characteristic curve J. In contrast thereto, the characteristic of the strip produced by the conventional method is degraded at the final annealing step for removing the strain from the copper and improving the bond, as indicated by curve K. According to the present invention,

since no such annealing is required, the characteristic obtained under the conditions of the heat treatment, conducted before casting the molten metal, can be retained as such, or since the heat treatment for improving the characteristic of the wire may be complemented by the heat of the molten metal cast, the characteristic degradation as encountered in practicing the conventional method will not occur. Strips which are produced in the same manner as that of characteristic curve .I but using in, Sn and Pb as a stabilizing metal at a temperature below 350? C, instead of Cd, have characteristics which are substantially the same as that represented by characteristic curve .I.

The chart of FIG. 7 shows the H-Ic characteristic curve of a superconducting strip produced according to the present invention using a wire of Nb-5Zr-60Ti ternary alloyfln the chart, curve L represents the characteristic curve of the wire of Nb-5Zr-60 Ti before heat treatment; curve M represents the characteristic curve of a strip produced by using a tape of the material which has previously been subjected to heat treatment at 500 C for 60 minutes and casting molten tin at 300 C', and curve N represents the characteristic curve of a strip which is produced by subjecting the wire of characteristic curve L to heat treatment at 500 C for 60 minutes beforehand, embedding said wire in a copper strip by rolling and thereafter annealing the strip at 500 C for 10 hours, according to the conventional method.

In the above-described embodiments wherein metals, such as Sn and Cd, which are relatively low in melting point are used as a stabilizing metal, the period of cooling after casting is not particularly critical. The strip solidified in the mold may be left to cool in a nonoxidizing atmosphere.

in the production of superconducting strips according to the method of this invention using a wire of Nb- Zr binary alloy type or Nb-Zr side Nb-Zr-Ti ternary alloy type superconducting material and Pb, Sn, In or Cd as a stabilizing metal, it is generally preferable to subject said wire to heat treatment in the temperature range from 450 to 700 C for less than l0 hours at lower temperatures and for about 30 minutes at higher temperatures beforehand. When aluminum is used as a stabilizing metal, the final heat treatment may be accomplished simultaneously with casting, by casting molten aluminum at 700 to 800 C over a non-heat treated wire, maintaining the strip thus formed in the temperature range from 600 to 700 C for a predetermined period within 60 minutes and thereafter allowing the strip to cool. The same is substantially true with a wire of Nb-Ti side Nb-Zr-Ti alloy type superconducting materials but, in this case, the heat treatment before casting is preferably carried out in the temperature range from 300 to 600 C for a period ranging from about hours for lower temperatures to about 5 minutes for higher temperatures. In practicing the method of this invention, when use is made of the heating of a wire by a molten stabilizing metal cast as the heat treatment for improving the superconducting characteristics of the product strip as described above, the conditions for such heat treatment can be set up as desired to some extent by adjusting the length of the mold used, the velocity at which the strip is drawn from said mold and the flow rate of cooling water or air circulating through the cooling jacket on said mold.

The method of the present invention is also effectively applicable to compound-type superconducting materials. For producing a coil with a wire of such compounds as Nb Al or Nb Sn which is very fragile mechanically, the compound is frequently shaped into a very thin tape and coated with the aforementioned stabilizing metal to form a strip, so as to prevent breakage of the wire in the process of the coiling operation. Therefore, the conventional method of producing a superconducting strip, in which a stabilizing metal is mechanically coated on a wire, is not adapted for the production of a strip comprising a wire of compoundtype, superconducting material, because there is more danger of wire breakage than in the case of producing a strip comprising a wire of alloy-type superconducting material. According to the present invention, however, breakage of a wire of compound-type superconducting material during the process of producing a strip with said wire can be entirely eliminated as in the case of producing a strip with a wire of alloy-type material.

In the case of a wire of compound-type superconducting material, heat treatment is more effective in elevating the critical temperature Tc than in increasing the critical current lc.

The relationships between the heat treatment temperature and the critical temperature Tc of Nb sn and Nb Al respectively in subjecting said compounds to heat treatment for 1 hour, are as indicated by curves 0 and P in the chart of FIG. 8. As seen, an excellent critical temperature value Tc can be obtained when the heat treatment is effected at about 800 C or higher in the case of Nb -,Sn, while the critical temperature curve of Nb Al forms a peak in the range of 700 to 900 C of the heat treatment temperature.

In producing a strip comprising a wire of these compounds, by the method of this invention, metals, such as Cu, Au and Ag, which are relatively high in melting point, may be used as a stabilizing metal. lt should be noted, however, that since the compound-type superconducting materials as mentioned above are thermally unstable and tend to be decomposed or form a non-superconducting compound upon reaction with molten stabilizing metal used, when they are subjected to a high temperature, a wire of the compound-type should be cooled as quickly as possible after it is casted in a stabilizing metal of 'relatively high melting point. For instance, a wire of Nb Sn heated at l000 C will not substantially be decomposed when it is cooled at'the rate of about 100 C per minute but will partially be decomposed into Nb Sn and Nb Sn when cooled at the rate of about C per minute. v

7 Heat treatment of a wire of Y Ga to elevate the critical temperature thereof is suitably effected at 900 to 500 C for l to 50 hours, particularly preferably at 700 C for 20 to 30 hours, and such heat treatment is preferably carried out beforehand when said wire is to be used for the production of a superconducting strip, because the heat treatment of the compound will require an excessively long time.

The method of producing a superconducting strip of this invention has the following advantages: Namely,

l. Substantially no mechanical work is required during the production process. Therefore, breakage or local strain of a superconducting wire embedded in a strip can be eliminated and a strip having a uniform structure and a high performance can be obtained with ease.

2. A highly satisfactory bond can be obtained between a superconducting wire and a stabilizing metal because the superconducting wire is completely embedded in the stabilizing metal. For a satisfactory bond between the wire and the stabilizing metal, it is preferable that they are bonded with each other through the intermediary of a thin diffusion layer of a thickness as large as the atomic distance formed therebetween, instead of being merely in contact with each other. The method of this invention provides for the formation of such diffusion layer and the thermal and electrical connections between the two materials can be markedly improved as the molten stabilizing metal is in direct contact with the superconducting wire.

3. The final heat treatment of a superconducting wire, which has been necessary heretofore, can be partially effected by the heat of the molten stabilizing metal cast over the wire. ln addition, it is possible to eliminate the step of annealing which has also been required heretofore, subsequent to embedding of the superconducting wire in stabilizing metal, for removing the strain in said stabilizing metal. A strip produced according to this invention using a wire of alloy-type superconducting material has a greater critical current than obtainable heretofore.

4. A long strip can be produced on a continuous basis. Consequently, it is possible to simplify the production process drastically, to increase the production rate, to improve the available percentage as a consequence of the advantage set out in l) above, and to reduce the production cost remarkably.

The superconducting strips produced in the manner described herein demonstrate excellent performances when used in large superconducting magnets and transformers. Thus, the present invention is of great industrial advantage.

What is claimed is: l. A method of producing a superconducting strip comprising the steps of:

' said cast aluminum coated superconducting wire at a temperature in the range from 600 to 700C for a predetermined period of time of between 10 to 60 minutes; and then d further cooling said cast aluminum coated superconducting wire to room temperature to thereby obtain a superconducting strip and continuously drawing the resultant strip from said mold.

2. A method of producing a superconducting strip consisting of a superconducting wire made of at least one member selected from the group consisting of Nb- Zr binary alloy-type Nb-Zr side Nb-Zr-Ti ternary alloytype superconducting material embedded in a stabilizing metal, which comprises the steps of:

a. subjecting said superconducting wire to a pre-heat treatment at a temperature in the range from 500 to 700C for 30 to 60 minutes; then continuously passing said pre-heated superconcasting a molten aluminum metal at a temperature in the range from 700 to 800C into said mold and then cooling said cast aluminum and superconducting wire to room temperature within l0 minutes and continuously drawing the resultant strip from said mold. 

1. A method of producing a superconducting strip comprising the steps of: a. continuously passing a superconducting wire made of at least one member selected from the group consisting of Nb-Zr binary alloy-type and Nb-Zr side Nb-Zr-Ti ternary alloy-type through a mold defining therein a passage having the same transverse cross-sectional shape as that of said superconducting strip; then b. continuously casting molten aluminum at a temperature in the range from 700* to 800* C into said mold; then c. applying a heat treatment to said superconducting wire by cooling said aluminum and maintaining said cast aluminum coated superconducting wire at a temperature in the range from 600* to 700* C for a predetermined period of time of between 10 to 60 minutes; and then d. further cooling said cast aluminum coated superconducting wire to room temperature to thereby obtain a superconducting strip and continuously drawing the resultant strip from said mold.
 2. A method of producing a superconducting strip consisting of a superconducting wire made of at least one member selected from the group consisting of Nb-Zr binary alloy-type Nb-Zr side Nb-Zr-Ti ternary alloy-type superconducting material embedded in a stabilizing metal, which comprises the steps of: a. subjecting said superconducting wire to a pre-heat treatment at a temperature in the range from 500* to 700* C for 30 to 60 minutes; then b. continuously passing said pre-heated superconducting wire through a mold defining therein a passage having a desired transverse cross-sectional shape then c. casting a molten aluminum metal at a temperature in the range from 700* to 800* C into said mold and then d. cooling said cast aluminum and superconducting wire to room temperature within 10 minutes and continuously drawing the resultant strip from said mold. 